1. Field of the Invention
The present invention relates to an optical pickup apparatus that records information to and reproduces information from an optical recording medium at high speed while maintaining photo-detection performance. Further, the present invention relates to an optical disk drive including the optical pickup apparatus.
2. Description of the Related Art
As disclosed in “Development of 7.3 mm Height DVD Optical Pickup Using TWIN-LD; 7th Microoptics Conference Jul. 14-16 1999”, a monolithic two-LD optical pickup device is known. In this optical pickup device, a photodiode (PD) chip and a monolithic integrated optical circuit, including a 650 nm laser diode (LD) and a 780 nm laser diode (LD), are packaged into a single optical module. The 650 nm LD emits a laser beam having the wavelength λ1=650 nm, which is used to play a DVD medium, and the 780 nm LD emits a laser beam having the wavelength λ2=780 nm, which is used to play a CD medium. The two-LD optical pickup device includes an optical system that focuses the laser beam, emitted by one of the 650 nm LD and the 780 nm LD, onto a recording medium using a common optical path. The PD chip detects the reflection beams that are reflected from the recording medium for each of the laser beams having different wavelengths.
In order to allow the single PD chip to receive each of the laser beams emitted by the two laser diodes having different emission points, the two-LD optical pickup device is configured to meet the relationship between an emission-point distance ΔL of the two laser diodes and an LD-to-PD distance L: ΔL=((λ2-λ1)/λ1)×L. Specifically, the two-LD optical pickup device has the emission-point distance ΔL=0.24 mm, and the LD-to-PD distance L=1.2 mm.
However, the optical system of an optical pickup device, which requires high-precision beam matching, must have an adequately small focal length of the collimator lens. The emission-point distance ΔL of the above optical pickup device is too large to meet the requirement. It must be reduced to about 0.1 mm in order to suit the above optical pickup device for the high-precision beam matching. If the focal length of the collimator lens is small but the emission-point distance is large, the diverging angle of the laser beams collimated by the collimator lens is large and the collimated laser beams are obliquely incident to the objective lens. This will make the focusing of the laser beam onto the recording medium inappropriate.
In the above-described optical pickup device, when the emission-point distance ΔL is set to about 0.1 mm, the LD-to-PD distance L is nearly equal to 0.5 mm. The LD-to-PD distance is too small to ensure high-speed operation of the PD chip. The heat generated by the two LDs will influence the PD chip, and the highspeed operation of the PD chip will be difficult.
Japanese Laid-Open Patent Application No. 9-120568 discloses a hybrid two-LD optical pickup device provided with a single optical module in which a 650 nm LD, a 780 nm LD and a PD chip are packaged. This optical pickup device uses a single hologram for allowing the recording and reproducing of several recording media. The positional accuracy of the emission points of this optical pickup device is not as high as that of the above-mentioned monolithic LD circuit. But, the laser diodes having the output performance needed for the optical pickup device can be selected, and the chip yield is better than that of the above monolithic LD device.
In the case of the optical pickup device disclosed in Japanese Laid-Open Patent Application No. 9-120568, it is difficult to perform the adjustment of the single hologram so as to make the offset of the two laser wavelengths small. This will raise the cost. Further, it is very difficult to maintain high accuracy of the packaging of the 650 nm LD, the 780 nm LD and the PD chip.
Moreover, as disclosed in “DVD/CD Optical Pickup Using Integrated Module Having Red/Infrared Laser Diodes and Photodetector; 47th Applied Physics Conference March 2000”, a hybrid two-LD optical pickup device in which the 650 nm LD and the 780 nm LD are arranged in parallel is known. A CD hologram and a DVD hologram are separately provided and they are adjusted independently. In this optical pickup device, the LD-to-LD distance is set to 1.1 mm, and the 650 nm laser beam does not pass through the CD hologram. The emission-point distance is 1.1 mm, and an optical axis compensating prism is provided in the optical system so that the optical axes of the 650 nm and 780 nm laser beams are suitably coupled.
However, in the above-mentioned optical pickup device, the optical axis compensating prism is expensive and large in size. In the optical pickup device that records information to an optical disk, the focal length of the collimator lens must be small. There is a problem in that the optical axis compensating prism cannot be arranged in such an optical pickup device.
As described in the foregoing, the hybrid optical pickup device is more appropriate, in optimizing the Photo-detection performance to suit for the specifications of the optical disk drive, than the monolithic optical pickup device. In addition, the hybrid optical pickup device has the advantageous feature that it is suitable for creating a low-cost, high-performance optical disk drive.
On the other hand, the hybrid optical pickup device requires separate positioning of the laser diodes onto the optical module, and an assembly error of the laser diodes mounted on the optical module is likely to occur, and the accuracy of the emission-point distance of the laser diodes is relatively low.